1. Field of the Invention
The present invention relates to a semiconductor memory device, and more particularly to a local sense amplifier in a semiconductor memory device.
2. Description of the Related Art
Semiconductor memory devices are devices in which data can be stored and from which the stored data can be retrieved. Semiconductor memory devices can be classified into random access memory (RAM) and read only memory (ROM). RAM is a volatile memory that needs power supply to retain data. ROM is a nonvolatile memory that can retain data even when power is removed. Examples of RAM are a dynamic RAM (DRAM) and a static RAM (SRAM). Examples of ROM are a programmable ROM (PROM), an erasable PROM (EPROM), an electrically erasable PROM (EEPROM), and a flash memory.
A semiconductor memory device comprises a cell array with a plurality of memory cells. Each of the memory cells is connected to a wordline and a bitline. By asserting the appropriate wordline and bitline, the data stored in a memory cell can be read and the data can also be written into the memory cell. When the memory device performs a read operation, the data stored in the memory cell is transmitted to the associated bit line. Next, a sense amplifier amplifies the data and transmits the data outside the memory device. Typically, the data transmission path includes a bit line pair BL and BLN, a local data line pair LDL, LDLN, a global data line pair GDL and GION, and an input/output line pair IO and ION.
With the increase in the number of memory cells, the physical area of the memory device increases, and so does the physical length of the data transmission path. For transmitting data stored in the memory cell quickly and completely, the sense amplifier should have enough driving ability. However, the driving ability of the sense amplifier is determined by the impedance of the data transmission path including an inherent line resistance, an inherent line capacitance and the capacitance of the pass transistors connected between the sense amplifier and the data path. Therefore, with the increase of the data transmission path, the sense amplifier is unable to maintain the high operating frequency of the memory device, and thus additional driving circuits are required to improve the transmission speed of the data.
U.S. Pat. No. 7,002,858 provides a semiconductor memory device in which a local input/output line sense amplifier can be selectively enabled or disabled. The semiconductor memory device comprises a redundancy circuit, a switch unit, and a control unit. If the redundancy circuit operates, the control unit generates a sense amplifier operation control signal that disables the local input/output line sense amplifier. The above memory device requires additional circuits and area to implement the redundancy circuit. U.S. Patent Publication No. 20090086550 provides a semiconductor memory device including a plurality of banks stacked in a column direction, a global data line and a common global data line driving unit. The above patent provides the semiconductor memory device that can minimize circuit dimensions for a data bus driving circuit in a stacked bank structure. However, it lacks a local sense amplifier and thus the operating frequency of the memory device may be limited.
Therefore, in order to increase the driving ability of the memory device, it is desirable to provide a memory device having a local sense amplifier implemented in a small layout area.